1. Field of the Invention
The present invention relates to a blank material to be fabricated into a drill (a blank material for manufacturing a drill) possessing a twisted cutting edge, and a drill fabricated from this blank material.
2. Prior Art
Hitherto, in order to meet the demands for enhancement of cutting performance or extension of tool life, polycrystalline diamond sinter (PCD) or cubic boron nitride sinter (CBN) which is very high in hardness and excellent in wear resistance has been used in cutting parts.
As this kind of cutting tool, as shown in FIGS. 9 and 10 for example, there is a tool having a cutting element 80 formed of sinter shaped in a thin sheet chip and brazed 81 to the peripheral corner of a base metal 8. However, due to the small adhearing area of the cutting element 80 on the base metal 8, the adhesion strength by brazing is insufficient and heat is generated during cutting, and these cause the cutting element 80 to come off from the base metal 8 in a short period.
As a tool which has solved the above problem, a cutting tool produced from a blank material as shown in FIG. 12 or FIG. 13 has been known.
The former, shown in FIG. 12, comprises a superhigh pressure sinter 83 sintered and affixed only to the end portion of a cemented carbide base metal 82, and as shown in FIG. 11 a front end portion 84 is formed on the superhigh pressure sinter 83. In this tool the above problem has been solved. However, since a cutting part formed of superhigh pressure sinter 83 is provided only in the front end, the cutting performance is not particularly excellent.
The latter, shown in FIG. 13, comprises a twisted groove formed in a cemented carbide base metal 82 and filled with superhigh pressure sinter 83, the base metal 82 and the sinter 83 being sintered and affixed together into one body, and a twisted cutting edge is formed on the superhigh pressure sinter 83 under the process of manufacturing a drill. In order to fabricate this blank material into a drill, however, the end part of the blank material including the superhigh pressure sinter 83 must be cut off into the shape of a drill front end, process of which is very difficult. Furthermore this blank material can not be machined with the both ends supported and so it is very difficult to manufacture a drill of high precision.
To facilitate the machining of the end part of the latter blank material, it might be considered to extend the twisted groove only part of the way from one end toward the other end. But it would prove not easy if actually tried. That is, in this case the twisted groove is machined by using a disc-shaped grinding wheel of a certain diameter (for example, 150 mm), but the process by the grinding wheel makes the depth of the groove not constant in the end part (i.e. the groove becomes shallower as approaching the end), and as a result the thickness of the superhigh pressure sinter 83 becomes thin at the end side. Therefore, the area of this thin superhigh pressure sinter 83 can not have a cutting edge thereon, but only be part of the front end of a drill. Hence machining for forming the front end with this blank material proves difficult.